Structures which use high strength, light weight materials, and which are subject to considerable stress in their normal use, are susceptible to the development of fatigue cracks, particularly in the regions where the sheets of such materials are joined. One example of such a structure is a modern large aircraft, and as the development of the present invention was in part stimulated by the problem of crack detection in the fuselage of a large aircraft, the application of the present invention to this problem will be given some prominence in this specification.
The cause of a significant number of aircraft accidents has been identified as structural failure resulting from the development of fatigue cracks and faulted bonds in the aircraft fuselage, in the vicinity of the riveted connection of the metal sheets forming the fuselage. Thus it is now recognised that the detection of fatigue cracks in aircraft structures, as soon as possible after their formation, is a matter of critical importance, and there has been a considerable commitment of resources to the establishment of a reliable in-situ crack detection technique.
Several commercial crack detection systems are now available. Those systems include eddy current detection techniques, ultrasonic examination techniques and magnetic rubber applications. Unfortunately, each of those techniques is suitable for only a certain limited range of applications, since the sensitivity of each method depends on the geometry of the component being investigated and other factors. Optical techniques for crack analysis using phenomena such as photoelasticity, caustics, moire pattern observations and shearography have also been developed, but those optical techniques (apart from shearography) are laboratory analysis tools and are not readily able to be adapted to in-situ testing of riveted, bolted or bonded structures. Thus there remains a need for a reliable in-situ crack detection system for use in the testing of joins in aircraft fuselage structures.
The present invention, as noted above is also useful in the examination of composite materials, known generally as "composites". Composites comprising resin with fibres of boron, carbon, glass or the like are now being used regularly to construct objects and structures for which the combination of mechanical strength and light weight is desirable. An example of such an object is the rotor blade of a helicopter.
One problem with such objects is that faults in the manufacture of the composite, which may affect the structural strength of the object, are often not visible. Another problem, arising out of the use of such objects, is that, if they receive a substantial blow during their normal use (for example, a bird strike on a helicopter rotor), the structural integrity of the composite material can be affected (that is, the object can be damaged) with no visible indication of the fact that the object has received a blow.
A particular problem experienced with such composite materials is that when they have been visibly damaged and have been patched (sometimes using as many as 30 layers of the patching composite material), there is no means of knowing whether the patch has been applied properly. A weak bond obtained during the patching process can result in the catastrophic failure of an object made from the composite material, without any warning. Similarly, there is currently no satisfactory technique available to test whether a boron composite patch, applied to an aluminium structure (for example, in the repair of an aircraft fuselage), has bonded properly to the substrate material.
One technique that has been used to test the structural integrity of objects made from such composite materials is the technique known as "shearography". This technique is described in some detail in the paper by Y Y Hung entitled "Shearography: A Novel and Practical Approach for Nondestructive Inspection", which was published in the Journal for Nondestructive Evaluation, Volume 8, 1989, pages 55 to 67. It is also described in the paper by S. L. Toh, H. M. Shang, F. S. Chau and C. J. Tay entitled "Flaw Detection in Composites using Time-average Shearography", which was published in Optics & Laser Technology, Volume 23, 1991, pages 25 to 30.
Briefly, shearography involves the imaging of a small region of the surface of an object being tested on a photographic film or plate which is positioned at an image plane that is remote from that region. The region is illuminated by the expanded beam from a laser. The image is produced from the reflection of the laser light using an "image-shearing camera", which produces a pair of laterally sheared images in the image plane. These images overlap and produce a speckled random interference pattern over the image. When the object is deformed (for example, by vibration), the displacement of the reflected, interfering beams modifies the speckle pattern. Toh et al, in their aforementioned paper, show that if the object is continuously vibrated, a time averaged "shearogram" is produced in the photographic emulsion. The shearogram can be reconstructed using a white light source. If there is a flaw in the region of the object under investigation (for example, a crack), the image of the shearogram is noticeably different over the region of the flaw, provided the object has been vibrated at approximately the resonance frequency of the flaw (which is invariably a much higher frequency than the natural resonance frequency of the object). While the technique described by Toh et al is no doubt capable of showing the presence of a flaw which is suspected, it is a cumbersome and time-consuming technique, requiring the production of shearograms for a range of frequencies. As Toh et al point out in their paper:
"As in holographic interferometry, the time-average technique in shearography is only applicable to steady state vibration studies". PA1 (i) for an effectively riveted or bolted join, PA1 (ii) for a join in which a rivet or bolt is present but serves as a pin rather than as a clamp, and PA1 (iii) for a join which has been stressed to the point where a crack has formed in the material lying under the domed head of a rivet or the head of the bolt PA1 are different from each other, but are consistent in their differences so that the quality of the join can be assessed from the features of the interference pattern. PA1 (a) mounting using vibration isolating pods, a photographic plate or film over, but spaced a small distance from the region; PA1 (b) illuminating the region with an expanded beam of a laser for a predetermined period, the laser beam passing through the plate or film to reach the region and be reflected from it, the reflected beam also passing through the plate or film; PA1 (c) applying stress to, or changing the stress applied to, the region so that there is a slight distortion of the region caused by the change in the stress thereof; PA1 (d) illuminating in the same manner a steps (b), the region and the photographic plate or film with the same expanded laser beam for a second predetermined period, whereby a holographic interferogram is created within the emulsion of the photographic plate or film; and PA1 (e) comparing the fringes of the holographic interferogram thus obtained in the emulsion of the plate or film with the fringes of a holographic interferogram from a correctly riveted, bolted or bonded join, undamaged or non-faulted composite material, properly bonded multi-layer patch, or normal state of the region, as the case may be, to determine the presence of any structural defect in the region.
In addition, if a region of a composite object which may be damaged is being investigated, the failure to detect a flaw does not necessarily mean that no flaw is present, unless observations of that region have been carried out over a wide range of closely-spaced vibration frequencies.
The holographic interferometry technique referred to by Toh et al is described in some detail in the paper by D. B. Neumann and R. G. Penn entitled "Off-table Holography", which was published in Experimental Mechanics, June 1975, pages 241 to 244. That technique utilises a laser with a spatial filter to illuminate a photographic plate which is attached to, but is spaced a short distance from, the surface of a solid structure. A diffraction pattern is recorded in the emulsion of the photographic plate. The diffraction pattern is produced by the interference of the illuminating laser beam and that portion of the illuminating beam which has passed through the plate and has been reflected from the region of the surface over which the plate has been positioned. If a second exposure is taken on the same photographic plate at a different stress level of the object, the two diffraction patterns combine to give a fringe pattern when the developed emulsion is observed in appropriate lighting.
The paper by Neumann and Penn illustrates the use of the holographic interference technique to show structural deformations in thick vibrating structures. It draws attention to the relatively low cost of such an analysis technique. However, it stresses the need for the surface that is being examined to be coated with a retro-reflective paint or tape, which restricts the uses to which the technique can be put. Indeed, Neumann and Penn effectively limit the application of their technique to an analysis of the deformations observable in large objects which have vibratable surface areas.